I. Field of the Invention
The present invention relates to a vascular anastomosis apparatus for anastomosing a blood vessel or the like with a laser beam.
II. Description of the Prior Art
Anastomosis of a blood vessel, a nerve, a lymphatic vessel or the like has been widely practiced in cardiac, cerebral and orthopedic surgery. Conventionally, a needle and thread are mainly used in suturing. However, according to this conventional operation, a small vessel having a diameter of less than 1 mm must be sutured more than 10 times to a portion surrounding the sutured portion of the vessel ends under microscopic observation, so that a highly experienced physician is required. It often takes one hour or longer to complete one suture.
FIG. 1 shows a structure of a blood vessel having an inner layer k, an intermediate layer l, and an outer layer m. When such a blood vessel is anastomosed with a needle and thread suture, the thread extending through the three layers physically damages the blood vessel. Thereafter, after anastomosis is performed, the tissue takes several weeks to recover. Complications such as thrombosis may occur. Also, a space is formed around the anastomosed portion, so that blood leaks from this portion as it flows, and it takes a long time for the blood to coagulate. This occurs typically in arterial anastomosis.
An experiment has been reported wherein optical energy was used to perform anastomosis in order to overcome these problems. The optical energy used was a type of radiation energy, and was expected to leave the organic tissue substantially undamaged as compared with anastomosis using a needle and thread. According to the report (Journal of Microsurgery, 436(1980)) by K. K. Jain, a polyethylene tube was inserted in a blood vessel, and a YAG laser beam was used to irradiate a portion to be anastomosed.
A YAG laser beam has a wavelength of 1.06 .mu.m and its power is fully transmitted to the tissue. This laser beam is not substantially attenuated in power since it is not substantially absorbed in the outer layer of a blood vessel. Therefore, it is considered that the YAG laser beam has the same energy at the inner layer of a blood vessel as at the outer layer thereof. In order to properly anastomose the blood vessel, the energy density of the laser beam at the inner layer thereof must be sufficiently decreased as compared with that at the outer layer thereof. This indicates that the laser beam must be focused on the outer layer by using an optical system having a short focal length. As a result, the operational procedures become time-consuming, and a highly experienced physican is required, resulting in inconvenience.
The mechanism of anastomosis using optical energy is not yet completely understood, but it is considered that collagens contained in the blood vessels melt and adhere to each other. This indicates that the laser beam is used as thermal energy. From this point of view, the conversion efficiency of the laser beam in converting optical energy to thermal energy is the most important factor in anastomosis using optical energy. Therefore, it is very important to use a laser beam having a large absorption coefficient with respect to organic tissue and water. For example, a carbon dioxide laser is known to have an absorption coefficient which is more than 1000 times that of the YAG laser. In organic tissue, all energy components of a carbon dioxide laser are converted to thermal energy at a depth of 0.1 mm from the skin surface. Therefore, when a carbon dioxide laser is used, a blood vessel can be anastomosed from the vicinity of the outer layer of the blood vessel without damaging the inner and intermediate layers. The carbon dioxide laser is expected to greatly improve the anastomosis effect. In addition to this advantage, unlike the YAG laser beam, the carbon dioxide laser has no restriction on the focal length, thereby improving operational procedures.
FIG. 2 is a graph showing the laser beam density as a function of the beam spot so as to explain the possible anastomosis conditions when a blood vessel having a diameter of less than 1 mm is anastomosed with a carbon dioxide laser. In this case, the scanning rate is 0.6 mm/sec, the lens is a ZnSe meniscus lens, and the incident beam spot diameter is 6 mm. Since capillary anastomosis is generally performed under microscopic observation, a practical focal length range is 5 to 30 cm. In the example shown in FIG. 2, when the focal length is 6 cm, the converged beam spot diameter is 0.1 mm. Under these conditions, an obtainable anastomosis energy or power density falls within the range of 1.9 to 4.5 W/mm.sup.2, which can be calculated to correspond to a laser beam energy range of 15 to 35 mW. However, when the focal length is 25 cm, the converged beam spot diameter is about 0.3 mm, and an obtainable anastomosis power density falls within the range of 0.8 to 1 W/mm.sup.2, which can be calculated to correspond to a laser beam energy range of 55 to 75 mW. As is apparent from the above description, when the converged beam spot diameter is increased, the obtainable anastomosis energy density falls within a narrower range, so that stricter control of the laser beam is required. Furthermore, the above conditions greatly vary in accordance with the type of blood vessel or its state. A small optical output must be accurately and precisely controlled at each application. Therefore, a carbon dioxide laser vascular anastomosis apparatus must inevitably satisfy the following conditions:
(1) an optical system having a long focal length of not less than 10 cm PA1 (2) a small converted beam spot diameter of about 0.1 mm, and PA1 (3) highly precise control of an optical output of not more than 100 mW.
A high-power cutter and a laser knife are known as typical examples of a carbon dioxide laser application. These apparatuses require a high-power laser of 10 to 100 W or higher. However, the vascular anastomosis apparatus of the present invention requires laser energy which is 1/1000 that of the high-power laser described above. Therefore, a very low-power laser energy required for the apparatus of the present invention must be achieved with an entirely new concept.
The conventional drawbacks are described as follows.
When a capillary having a diameter of not more than 1 to 2 mm is subjected to vascular anastomosis, the laser spot must be located within the microscopic range of view of the microscope since capillary anastomosis must be performed under microscopic observation. However, in the conventional manipulator system or the conventional optical fiber system, the manipulator or optical fiber must be located in the vicinity of the object to be examined, even when the focal length of the laser beam is as short as 1 to 2 cm, thereby interfering with the view through the microscope. However, when the focal length of the laser knife apparatus is increased (i.e., is 10 cm or longer), a beam spot having a diameter of 0.1 to 0.2 mm or less required for capillary anastomosis cannot be obtained due to diffraction. These conventional drawbacks result from the fact that a laser beam having a considerably small beam (e.g. having a beam diameter of several millimeters) is incident on the focusing lens in the conventional laser knife apparatus. This can be analyzed from the optics as follows.
When the diameter of the laser beam spot is given as a, the focal length of the focusing lens is given as f, and the wavelength of the laser beam is given as .lambda., a diameter b of the laser beam spot at the focal point is given as b=4 .lambda.f/a.pi. due to the diffraction phenomenon, even if the focusing lens is assumed to have no aberration. When the carbon dioxide laser is used, the focal length f is typically about 6 cm and the laser beam spot diameter a is typically about 6 mm in the conventional laser knife apparatus. Therefore, when the wavelength form .lambda. is 10.6 .mu.m, the spot diameter b becomes 0.1 mm. In this case, the spot diameter b falls within the possible operation beam spot diameter range of 0.1 to 0.3 mm. When the focal length f is equal to or greater than 20 cm so as to perform anastomosis under microscopic observation, the spot diameter b becomes about 0.4 mm. As a result, this diameter falls outside the possible operation beam spot diameter range, as shown in FIG. 2. Therefore, when the laser beam guided through the optical fiber is converged, no possible operation beam spot diameter can be obtained, since the diameter a is less than 1 mm.
As far as the energy density is concerned, the possible operation beam range corresponds to a very low power optical output range. For example, when the focal length f is 6 cm, an energy density falls within the range of 2 to 4.5 W/mm.sup.2. This energy density corresponds to a laser output of 15 to 35 mW. In the conventional laser knife, an incision, an evaporation process, and the like are performed in a laser power range of 10 to 100 W. When the high-power laser discharge tube is controlled to produce a small discharge current so as to obtain the anastomosis range, the discharge current must be decreased to fall within the range of 0.1 to 1 mA. In general, the discharge tube has a high impedance and often tends not to be controlled. Therefore, it is very difficult to perform anastomosis with the conventional laser knife.